Corrosion may be defined as the effects of unwanted chemical reactions on the surface of iron and steel. Corrosion causes deterioration of the surface and structural properties of the iron component. A substantial amount of the iron currently produced is used to replace deteriorated existing structures. Thus, there is a great demand for eliminating or, at a minimum, reducing corrosion of iron and steel.
One method to inhibit corrosion is to coat the surface of the substrate with a corrosion inhibiting additive. Typical examples of such additives for inhibiting the corrosion of iron and steel are the chromates, phosphates, and silicates. They frequently are effective with stainless steel and other alloys.
Corrosion, being an electrochemical phenomenon, can be tackled through the use of electrochemistry and conducting polymers. The common method of cathodic protection, just from a theoretical stand point, cannot be the right approach, because cathodic protection requires a higher current density which cannot be provided by a conducting polymer. Anodic protection on the contrary entails current density of the order of 10.sup.-3 to 10.sup.-6 amperes per cm.sup.2 which a conducting polymer is capable of imparting to the piece of iron or steel. The other important parameter is the electrical potential necessary to passivate iron or steel.
There has been recent interest in the prevention of corrosion of iron through the use of electro-active polymer coatings. Mengoli et al. performed anodic synthesis of sulfur bridged polyaniline coatings on to iron sheets by electrolyzing a basic solution of aniline and ammonium sulphide. The polymerization proceeded with avoidance of a side reaction that might lead to the contamination of the coating with azobenzene. Sulfur probably enters the polymer chains. Use of n-alkylaniline was superior because the coatings are thermally curable and show satisfying physical properties. The electropolymerization of aniline and ammonium sulphide was carried out at 4-5 V producing an insulating oligomeric film. The film was dried in an oven at 150.degree. C. leading to cross linking consequently making it insoluble in any solvent. The film showed salt fog corrosion resistance close to 80 hrs.
DeBerry and Viehbeck showed that in a corrosive environment, electroactive Prussian blue coatings can maintain suitable metals in a passive state and can act to repassivate damaged areas. DeBerry deposited polyaniline coatings on stainless steels 410 and 430 by electrolyzing 1.0M aniline in pH 1.0 perchloric acid solution by cycling the stainless steel working electrode potential between -0.2 and +1.1 v vs. SCE at a scan rate of 50 mv/s. This resulted in the deposition of polyaniline layer in each cycle. The metal was passivated at -0.27 V in the first cycle and polyaniline was deposited at 1.0 V. The cyclic voltammogram of the deposited film in the voltage range -0.2 to +1.1 V at a scan rate of 20 mV/S in 0.2M H.sub.2 SO.sub.4 showed three oxidation and three reduction peaks and the polyaniline showed reversibility of its redox behavior. The electron transfer between the iron metal and the polyaniline through the passive iron oxide layer was facile and partially responsible for the ability of polyaniline to maintain the passivity of the stainless steel. The method of corrosion monitoring was to dip the polyaniline coated stainless steel in the corrosive medium and measure open circuit voltage (Voc) vs. SCE, with time. The Voc of polyaniline coated stainless steel remained above that of bare stainless steel in aerated 2N H.sub.2 SO.sub.4 for at least 50 days without breakdown. In the presence of aggressive chloride ions (0.1N H.sub.2 SO.sub.4 +0.1M NaCl+0.15M Na.sub.2 SO.sub.4), polyaniline coated SS-430 lasted without pitting corrosion for more than 30 days. These coatings, applied by electrochemical deposition processes, are not applicable to the prevention of corrosion of most metal structures where electrochemical deposition is impractical.
The inhibition of corrosion on metallic substrates is a serious aspect of research due to the substantial economic loss associated with the corrosion of such substrates. There exists a current need to provide superior coatings and coating processes to inhibit such corrosion.